1. Field of the Invention
This invention relates to a process and apparatus for quantitatively determining the amount of hexavalent chromium (Cr.sup.+.sup.6) in a process stream. In particular, it relates to a process and apparatus for measuring small quantities of CR.sup.+.sup.6 in process streams containing a variety of other interfering components.
2. Discussion of the Prior Art
Hexavalent chromium has been listed by the Environmental Protection Agency (EPA) as one of the primary pollutants which must be controlled and monitored in waste water streams. A concentration limit of 0.05 ppm of hexavalent chromium in such streams has been proposed.
The standard method used to detect Cr.sup.+.sup.6 is to react Cr.sup.+.sup.6 with diphenylcarbazide to produce a reddish purple color in slightly acid solutions. This procedure, however, is not easily adapted to a continuous measurement.
Hexavalent chromium (Cr.sup.+.sup.6) absorbs strongly in the near ultraviolet (UV). Its presence in large concentrations, is therefore easily detected by UV spectroscopy. Its presence in small concentrations is also easily detected by UV spectroscopy, provided that there are no other impurities in the medium which would mask the presence of Cr.sup.+.sup.6. Direct photometric absorption measurements for Cr.sup.+.sup.6 in the part per billion/part per million range in plant effluent waters, however, have not been used because the varying absorption of other impurities in the water usually exceeds that of the Cr.sup.+.sup.6.
There is a need, therefore, for a simple, accurate technique to continuously monitor process streams for the presence of Cr.sup.+.sup.6 in amounts commensurate with EPA levels.
The present invention uses a sulfur dioxide reducing agent, preferably sulfur dioxide gas, to reduce Cr.sup.+.sup.6 to trivalent chromium (Cr.sup.+.sup.3) and provide a reference sample to compensate for background absorption. SO.sub.2 gas has been used for the abatement of chromium in plant effluent waters, by reducing Cr.sup.+.sup.6 to Cr.sup.+.sup.3 and then precipitating Cr.sup.+.sup.3 by making the stream basic. However, SO.sub.2 has never been used in the analysis of a process stream for Cr.sup.+.sup.6 ; nor has it been used to produce a reference sample from a plant stream which can, in turn, be used to provide an accurate determination of the Cr.sup.+.sup.6 content of that stream. It is the use of SO.sub.2 to produce a reference sample which makes such an accurate determination possible, and it is not readily apparent, from the use of SO.sub.2 in abatement procedures, that such a reference sample will permit accurate measurements of the Cr.sup.+.sup.6 content in process streams, particularly when that stream contains small concentrations (in the ppm range) of Cr.sup.+.sup. 6 in combination with high concentrations of other interfering materials.